The Facts Around the Leaks
When implementing a drainage system for the recovery of food residues, an assembly of CPVC (Chlorinated Polyvinyl Chloride) was used. Three months after the commissioning of the building, leaks were observed in the pipes and elbows of the assembly. Certain parts of the assembly (specimens) were therefore subjected to an investigation to determine the most probable cause of failure (Figure 1).
The investigation was carried out by the undersigned and the results obtained were published on September 9, 2020, in the digital edition of the Journal of Failure Analysis and Prevention. The following is a summary of this investigation and its results.
Figure 1 – Specimen of the assembly (pipes and elbows) of a drainage system for the recovery of food residues.
(Source: Photo taken by Technorm)
Tests and Observations
The specimens received were parts of pipes and elbows with cracks visible on their outer surfaces. Inside the specimens, traces of mould were visible. While cleaning the inner surface, we observed that the cracks were going through the wall of the elbow. The cracks appeared to be initiated by the cement joining the elbow and the pipe (Figure 2). In an effort to identify the cause of the failure, the cracks were opened and observed at high magnification using a scanning electron microscope (SEM). Chemical analyzes of the contents of the pipes were also carried out.
Figure 2 – Inner surface of the assembly showing cracks visible on (a) both sides of the elbow and along the cement weld and (b) the internal side of the assembly along the cement weld.
(Source: Photo taken by Technorm)
Several Factors InvolvedSeveral Factors Involved
The results of the high magnification study (SEM) demonstrated that the cracks initiated inside the elbow and propagated to the outer surface by a creep phenomenon (Figure 3). Creep is the tendency of a material to deform permanently under the influence of constant stress. The residual stress during the manufacture of the part can be sufficient for the creep phenomenon to occur. Usually, this type of failure is slow and progressive. However, it can be accelerated by the presence of products incompatible with plastic. This is called environmental stress cracking (ESC).
Chemical analyzes of the contents of the pipes revealed the presence of numerous products incompatible with CVPC. These were mainly compounds formed during the decomposition of food residues inside the pipes. Cracks were visible at the junction between the elbow and the pipe, where the two parts are fused together via the application of cement. Cement can itself be an ESC agent.
According to the installation of the CPVC assembly, areas facilitating the build-up of food residue were present in the elbows near the cement. Since cement is a spongy material, it can absorb and trap contaminants. The cement on the inside of the elbow was in constant contact with the food residues flowing inside the pipes and so became permeated with products incompatible with CPVC. The accumulation of food residues increased the duration and frequency of exposure to incompatible products which eventually affected the strength of the CPVC until it cracked after only 3 months.
Figure 3 – Initiation of one crack on the inner surface of the elbow near the cement.
(Source: Photo taken by Technorm)
Conclusion
The results of our investigation revealed numerous cracks at the junction between the elbow and the pipes, near the cement. The cracks started from the inside of the elbow and spread to the outside surface of environmental stress cracking (ESC). Environmental stress cracking (ESC) is caused by the simultaneous effect of the following three factors:
- a material susceptible to ESC,
- a tensile stress, and
- an ESC agent (combination of chemicals from the decomposition of food residue and solvent from the cement weld).
The choice of material when the drainage system was designed for the recovery of food residues was therefore inadequate. The build-up of food residues in the assembly contributed to premature failure. In the event that there had been no accumulation zone, the failure would still have occurred, but probably after a longer period of service.
To learn more
Technorm may have the right training for you. Discover our training offer.
Questions on the subject? Contact the author of the article!
By Jane Gagné, chemist, Mechanics and materials team
